Injuries that trigger loss of small vessels and capillaries in the adult lung, and wall overgrowth in ones that survive ultimately restrict blood flow and increase vascular resistance and pressure to cause pulmonary hypertension (PH). The long-term goals of the present studies are to obtain data for circulating blood-derived cells that have the potential to form new vascular units in the adult lung, and the RTK-ligand signaling systems that regulate blood vessel formation, as a rational basis for strategies to promote (re)growth of vascular segments and block wall overgrowth. Our recent data in an in vivo model of vascular remodeling in PH indicate that the FiO2 triggers regression and growth of vessel and capillary units to match vascular density to the functional demands of the tissue. Because vessels of the normal alveolar-capillary membrane form the continuation of capillary networks many are simple endothelial-lined channels with few, if any, mural cells. As hyperoxia (FiO2 0.87) prunes these segments, mural cells develop around ones that survive, resulting first in wall support and then in wall overgrowth. The early stage of return to air (representing relative hypoxia to the hyperoxia-adapted lung) triggers further mural cell development and the formation of new vessels and capillaries throughout the alveolar-capillary membrane. Based on preliminary data, the hypotheses tested are that circulating endothelial progenitors (CEPs) form these new vascular networks, and that FiO2 -triggered increase in expression of RTKs for PDGF (PDGFR-alpha and R-beta) by interstitial fibroblasts and ANG- 1 (TIE-2) by local endothelial cells induces mural cell development, and increase in expression of RTKs for VEGF and ANG-2 (VEGFR-1, VEGFR-2 and TIE-2) by CEPs and endothelial cells the formation of vascular units. Blood-derived cells will be characterized by cell lineage analysis. The requirement of PDGFR-beta for mural cell development will be demonstrated by function-inhibition antibody studies. FiO2--induced vascular cell regression, unit pruning, by apoptosis, will be assessed by cell death assays, and the protection afforded by VEGF investigated. Quantitative data will be obtained by state-of-the-art high resolution immunogold labeling and digital imaging techniques for (i) fluorescein labeled/luciferase transfected blood-derived cells and (ii) RTK and ligand (PDGF-AA, PDGF-BB, VEGF, ANG-land ANG-2) expression levels in endothelial and mural cells of forming vascular units in growth responses triggered by the FiO2. These approaches will demonstrate a role for blood derived cells and RTK signaling triggered by an 'abnormal' FiO2 that lead to the formation of new vascular units and wall overgrowth, and provide data on which to base therapeutic approaches to prevent or correct the vascular remodeling that results in severe illness in human patients.